Study on geochronology, rock genesis and tectonic background of Luocheng granodiorite in Longshou Mountain tectonic belt

The geotectonic location of Longshou Mountain Metallogenic Belt of the Alxa Massif is at the junction of the North China Plate, the Qilian Orogenic Belt, and the Central Asian Orogenic Belt, and the particularity of the tectonic environment is of great significance to the regional tectonic evolution and plate movement. The magmatic evolution and tectonic background of this area are not well studied. The author has carried out studies on the rock geochemistry, zircon U-Pb dating, and Lu-Hf isotope of Luocheng biotite granodiorite. LA-ICP-MS zircon U-Pb dating shows a value of (289±3)Ma (MSWD=0.57), and the emplacement time of the rock is in the Early Permian. Luocheng Granodiorite is quasi aluminous, calc alkaline series, with Mg # value between 0.64-0.66, showing the characteristics of I-type granite. These early Permian Granodiorite in the study area show the characteristics of relative enrichment of light REE and relative depletion of heavy REE, LREE/HREE ratio was 4.20-5.30, and (La/Yb) _N was 3.69-5.46, And it has a slight negative Eu anomaly(δEu: 0.80-0.96). In the standardized diagram of the primitive mantle, In the standardized diagram of the original mantle, these samples are relatively rich in large ion lithophile elements (Rb, Th, K, etc.) and depleted in high field strength elements (Nb, Ta, Ti, P, etc.), showing the geochemical characteristics of subduction related arc magmas. The early Permian intrusive rocks in the Luocheng area have positive ε The Hf (t) value (+4.37 to+6.88) and the relatively young two-stage mode age (T _(DMC)=808.6 to 952.5 Ma). Field geological characteristics and geochemical data show that Luocheng Granodiorite is the product of mixing of crust derived magma and mantle derived magma. Based on the analysis of regional background and foreword research data, it is possible that arc magmatic activity related to subduction existed in the Longshoushan tectonic belt in the western part of Alxa during the Early Permian.